What are `d`-block elements? Why are they called transition metals?

Explanation

To answer the question, "What are `d`-block elements? Why are they called transition metals?", we can break it down into two parts: defining `d`-block elements and explaining why they are referred to as transition metals.

### Step-by-Step Solution:

1. **Definition of `d`-block Elements**:
- `d`-block elements are the elements found in groups 3 to 12 of the periodic table.
- These elements have their valence electrons in the `d` orbital.
- The general electronic configuration of `d`-block elements can be represented as \( (n-1)d^{1-10} \, ns^{0-2} \), where \( n \) is the principal quantum number. This means that they can have 1 to 10 electrons in the `d` orbital and 0 to 2 electrons in the `s` orbital.
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